Metal dielectric composite resonator

ABSTRACT

A composite resonator ( 10 ) consisting of a conducting metal ( 14 ) and a dielectric material ( 12 ) is used to provide resonant frequencies lower than can be obtained using the same volume of dielectric alone and with higher unloaded Q than can be obtained using the same volume of metal imbedded into a cavity and used as a resonator. This significantly reduces the cost and size of the resonator ( 10 ) without degrading its performance. An inexpensive metal ( 14 ), such as aluminum, can be substituted for more than half of the dielectric ( 12 ) and stille form a resonator ( 10 ) with substantially equivalent resonant properties. The operative embodiments of the resonator invention ( 1 ) cover composites with doughnut-shaped, i.e., cylindrical, configurations, with the “doughnut” either metal ( 14 ) or dielectric ( 12 ), and the “hole” either dielectric ( 314 ) or metal ( 312 ), respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a resonator composed of a conducting metal ringsurrounding a cylindrical dielectric core material which can beincorporated into multi-cavity filters for frequency separation.

2. Description of Related Art

Dielectric resonator filters are a class of stable microwave filtersthat are frequently used in radar and communications systems. Dielectricresonators are often utilized in filter circuits because of anintrinsically high Q value. These characteristics allow a filteremploying a dielectric resonator to have excellent frequency stabilitywith only a small amount of frequency drift over a wide range oftemperatures and environmental conditions. The Q value of a dielectricresonator is defined as the ratio between the energy stored per cycle tothe energy dissipated per cycle.

Dielectric resonators are typically made of a ceramic type materialhaving a high dielectric constant (∈_(r)=20 to 90) and a low dissipativeloss. These characteristics allow the dielectric resonator to storeenergy with relatively little internal energy dissipation. Thiscorresponds to a high Q value.

One significant limitation of the practical use of dielectric resonatorfilters is the cost of the dielectric itself. The cost of a typicalprior art 6″ ceramic dielectric cylindrical resonator can cost threehundred dollars or more. In addition, the size of the resonatorsubstantially increases the size of any multi-cavity filter in which itmight be employed.

The following patents are generally representative of typical prior artdielectric resonators: U.S. Pat. Nos. 4,757,289; 5,140,285; and,5,356,844.

Resonators are typically employed in filters for the wirelesscommunication industry. Such filters typically include a plurality ofresonators located in adjacent cavities and coupled to each otherthrough a variety of different means. One coupling mechanism known inthe prior art is the use of a tunable iris as described in U.S. Pat. No.5,220,300 entitled “RESONATOR FILTERS WITH WIDE STOPBANDS” and issued onJun. 15, 1993 and assigned by Richard V. Snyder to RS Microwave Company,Inc., the entire contents and substance of which is incorporated hereinby reference. Other cutoff means are also known, but few are known thatwould be suitable for composite resonators such as described in thisdisclosure.

What is clearly missing in the prior art, therefore, is a relativelyinexpensive resonator, of reasonably small size, that can be used in amulti-cavity filter structure without appreciable loss in performance.

SUMMARY OF THE INVENTION

Briefly described, the invention comprises a composite resonatorpreferably including a cylindrical ceramic core and an exterior metallayer that surrounds most of the exterior circumference of the core andwherein the resonator resonates in substantially bound modes. Thiscomposite configuration is used to provide resonant frequencies lowerthan can be obtained using the same volume of dielectric alone and withhigher unloaded Q than can be obtained using the same volume of metalimbedded into a cavity and used as a resonator. An inexpensive metal,such as aluminum, can be substituted for more than half of thedielectric and still form a resonator with substantially equivalentresonance properties.

According to alternative embodiments of the invention, the resonatorsare incorporated into spectrum filters for separation of frequencies. Ascontrasted to .conventional prior art implementations, the new techniqueachieves similar, or better, electrical performance; similar, orreduced, size; and significantly reduced cost for applications in thefrequency range below 2.5 Ghz, thus including PC, wireless, AMPS and GSMapplications, as well as a myriad of other applications in thisfrequency range. With spectrum currently selling for up to $45.00 perHz, filters are very valuable for providing users the opportunity toutilize all spectrum available. Yet, the cost of the filters mustultimately be borne by the users, so reductions in cost are important tocommercial applications. The present invention in its variousembodiments contributes to such a reduction in cost.

These and other features of the invention will be more fully understoodby reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art ceramic resonator core.

FIG. 2 is a perspective view of a prior art multi-cavity filterincluding four prior art ceramic resonators.

FIG. 3a is a perspective view of a composite resonator that comprises adielectric core surrounded by an exterior metal layer according to thepreferred embodiment of the invention.

FIG. 3b is a vertical cross-sectional view of the preferred embodimentof the resonator shown in FIG. 3a.

FIG. 3c is a top cross-sectional view of the resonator according to thepreferred embodiment a illustrated in FIGS. 3a and 3 b.

FIG. 4 is a perspective view of one embodiment of the invention showingthe metal dielectric composite resonator according to the preferredembodiment utilized in a basic filter resonator having a ferrite orgarnet disk, magnetically tunable in frequency.

FIG. 5 is a perspective view of an alternate embodiment illustrating twometal dielectric composite resonators according to the presentinvention, each with a window facing one another in their metallizedcircumference, thereby permitting coupling of energy between the tworesonators.

FIG. 6 is a perspective view of another alternate embodiment of theinvention illustrating a coupled filter in which two metal dielectriccomposite resonators according to the present in vention are coupled bymeans of a tunable iris.

FIGS. 7, 8 and 9 are perspective views of other alternative, hybridembodiments illustrating cross coupled array filters in which four metaldielectric composite resonators according to the present invention arecoupled by various means.

FIG. 10 perspective view of another alternative embodiment illustratinga metal dielectric composite resonator according to the presentinvention employed in a dual mode filter configure.

FIG. 11a is a perspective view of an alternative embodiment of aresonator with a shape similar to the preferred embodiment shown inFIGS. 3a-3 c but having a metallic core and an external layer ofdielectric material surrounding most of the surface of the metalliccore.

FIG. 11b is a vertical cross-sectional view of the alternativeembodiment of the resonator shown in FIG. 11a.

FIG. 11c is a top cross-sectional view of the alternative embodiment ofthe resonator shown in FIGS. 11a and 11 b.

FIG. 12 is a resonator characteristic graph providing an example of howa typical resonator, according to the preferred embodiment of theinvention, is structured and designed.

DETAILED DESCRIPTION OF THE INVENTION

During the course of this description like numbers will be used toidentify like elements according to the different figures thatillustrate the invention.

FIG. 1 illustrates a typical prior art dielectric resonator. Such priorart resonators are typically relatively large and made of a singlematerial, such as ceramic. Because of their size they can be relativelyexpensive to manufacture. In addition, their larger size in turndictates that any filter in which they are used will also be relativelylarge, and thus display undesirable spurious responses in closeproximity to the resonant frequency of the resonator. See FIG. 2 for atypical prior art multi-cavity four-stage filter including four priorart dielectric resonators.

A composite resonator 10, according to the preferred embodiment of theinvention, is illustrated in FIGS. 3a-3 c. The preferred resonator 10includes a ceramic core 12 surrounded by a metal layer 14 to form a“doughnut” or “hockey puck” shape. The core 12 includes a top surface orface 22, a bottom surface or face 24, and an interior sidewall surfaceor face 26. As best seen in FIG. 3b, the circumference 26 of the core 12is surrounded by a sidewall metallic band or layer 18. The metalliclayer 14 is, of course, the side layer 18. The metallic layer 14, i.e.,ring 18, is preferably at least 2-3 skin depths thick or deep. Ring 18can be much thicker but must be at least 2-3 skin depths to operateproperly. The term skin depth is well known in the prior art and definedas {fraction (1/e)}.

FIG. 4 illustrates a relatively simple, basic embodiment 30 in which theresonator 10 is employed as a filter. The resonator 10, which includesthe core 12 and the surrounding metal layer 14, as described with regardto FIGS. 3a-3 b, is located in a structure, or housing, 32 and fed by aconventional probe 36 supported at anchor point 34. The ∈_(r), and μ canbe chosen to vary the characteristics of the resonator. The resonator 10comprises a ferrite or garnet disk, magnetically tunable in frequency.

According to the preferred embodiment of the present invention,composite resonators are used in a resonator apparatus that operates ina substantially bound mode. In a substantially bound mode, the signal isessentially contained within the high dielectric material and isessentially non-radiating. This is due to the almost perfect reflectingboundary conditions resulting from both the selective use of conductivemetallization on the periphery and the critical angle of reflection atthe non-metallized boundaries of high dielectric constant material(∈_(r)≧10, and typically ∈_(r)=24 or greater) with the low dielectric(∈_(r)=1) air filling the enclosures. What is important is the ratio ofdielectric constant filling the resonator to that filling the cavity,external to the resonator. To ensure almost perfect reflection and thusresonance of substantially bound modes, the ratio should be at least15:1. Examples of substantially bound modes function in this applicationare the TE_(oin) modes which exist substantially without leakage in thestructure described herein. In the example mode, the subscripts refer tothe number of circumferential, radial and longitudinal magnetic fieldvariations (for the case of a cylinder).

The invention 10 is not limited to round doughnut shapes, as theprinciple also applies to planar configurations or parallelepipedresonator configurations. The invention also applies to planarconfigurations in which metal dielectric composites are used to formartificial dielectric screens for application to antennas and similardevices.

Substantially bound modes become unbound only at specific interfaceswherein coupling mechanisms such as irises, tuning screws, or otherperturbations are present, and then only for purposes of enhancingcoupling of a portion of the substantially bound mode to anotherstructure such as another resonator or port. FIGS. 5-10 depict suchcoupling mechanisms in various combinations.

The foregoing invention is described primarily in the context of acylindrical example. It should be understood, however, that it canoperate in any of the recognized nine “separable geometries”. “Separablegeometries” is a term known in the prior art and is described, forexample, in “Methods of Theoretical Physics”, by Morse and Feshbach,McGraw Hill, 1953. The geometries, which are included in the nineseparable modes, are believed to be the only ones which can support morethan one orthogonal mode simultaneously.

FIG. 5 illustrates a filter embodiment 40 housed in a structure 42having a cavity 49 and a standard energy feed port 44. A first and asecond composite resonator 46 a and 46 b are attached at opposite endsof the cavity 49. The first resonator 46 a includes a small window 48 alocated in the metallic sidewall sufficient to expose the underlyingdielectric core 12. Similarly, the second composite resonator 46 bincludes a window 48 b in its metallic sidewall that faces window 48 aof the first resonator 46 a. Energy from the first resonator 46 a iscoupled through window 48 a to window 48 b of the second compositeresonator 46 b.

Another coupling embodiment 50 is illustrated in FIG. 6. Filter, orcoupling, embodiment 50 comprises a housing structure 52 that includes apair of cavities 60 a and 60 b. Energy is coupled into the cavity by astandard fitting 54. Cavity 60 a includes a composite resonator 56 awhich sits atop a pedestal support 58. Similarly, a second compositeresonator 56 b sits atop a pedestal 58 in cavity 60 b. In real life, allresonators 10 et seq. shown in FIGS. 4-10 sit on pedestals like 58 butare not shown because they are well known in the prior art. Suchpedestals, sometimes referred to as “toadstools”, typically have a low ∈(in range of 2-6) and are made of foam or B_(e)O. Partition, or wall, 62separates cavity 60 a from 60 b. A window 64 is located in wall 62 andincludes a tunable iris 66 for selectively coupling energy fromcomposite resonator 56 b to composite resonator 56 a. A tunableresonator having an acceptable iris structure is described in U.S. Pat.No. 5,220,300 issued on Jun. 15, 1993 and assigned by Richard V. Snyderto RS Microwave, Inc., Butler, N.J.

A cross-coupled array filter 70 embodiment is illustrated in FIG. 7. Thehousing structure 72 includes a standard energy port 74 and defines apair of interior cavities 78 a and 78 b. A first and a second compositeresonator 76 a and 76 b, respectively, are located within cavity 78 b.Similarly, a third and fourth composite resonator 76 c and 76 d arelocated within cavity 78 a. A partition, or wall, 82 separates cavities78 a and 78 b. A pair of windows 82 a and 82 b is located in partition82. Window 82 a includes a tunable iris 84 a. Likewise, window 82 bincludes a tunable iris 84 b. Tunable irises 84 a and 84 b can beidentical to those described in U.S. Pat. No. 5,220,330. Energy from thefirst composite resonator 76 a can be selectively coupled through iris84 a to the third composite resonator 76 c. Likewise, energy from thesecond composite resonator 76 b can be coupled through iris 84 b to thefourth composite resonator 76 d.

FIG. 8 illustrates another alternative embodiment 100 which is acombination, or hybrid, of the window and iris coupling mechanisms. Asillustrated in FIG. 8, the combination embodiment 100 is housed in astructure 102 and includes a standard energy coupling 104. Housing 102includes interior cavities 110 a and 110 b. Cavity 110 b houses a first,second, and fourth composite resonator 106 a, 106 b and 106 d,respectively. Cavity 110 a houses third composite resonator 106 c. Thesecond and fourth resonators 106 b and 106 d each include a window 108 band 108 d, respectively, which face each other and which couple energyfrom the second composite resonator 106 b to the fourth compositeresonator 106 d in the manner previously described with reference toFIG. 5. The interior cavity 110 a is defined by right angle panels orpartitions 112 and 114, respectively. Panel 112 includes a window 116and a tunable iris 118, similar to that described in U.S. Pat. No.5,220,300 for coupling energy from the first composite resonator 106 ato the third composite resonator 106 c. Similarly, panel 114 includes awindow 120, and a tunable iris 122, for selectively coupling energy fromthe fourth composite resonator 106 d to the third composite resonator106 c.

Another combination or hybrid coupling embodiment 150 is illustrated inFIG. 9. The resonators are located within a housing structure 152 whichincludes the standard energy port 154. Housing 152 defines a singleinterior cavity 160 which houses a first, second, third and fourthcomposite resonator 156 a, 156 b, 156 c and 156 d, respectively. Thesecond and fourth composite resonators 156 b and 156 d each includewindows, or apertures, 158 b and 158 d, respectively, which coupleenergy from the second composite resonator 156 b to the fourth compositeresonator 156 d. A partition, or wall, 162 separates the first compositeresonator 156 a from the third composite resonator 156 c. A window 164is located in the wall 162 and includes a tunable iris 166, similar tothat described in U.S. Pat. No. 5,220,300 for coupling energy from thefirst composite resonator 156 a to the third composite resonator 156 c.

FIG. 10 illustrates a single resonator 204, dual mode, configuration 200that takes advantage of the fact that substantially bound modes becomeunbound where perturbations are present. The resonator 204 has adistinctly rectangular shape and is located within housing 202. It isfeed energy by a conventional probe 206 and includes a rectangulardielectric core 208 partially, but not entirely, surrounded by ametallized peripheral layer 210. A three-dimensional orthogonal notch212 is taken out of one comer of the composite resonator 204. The notch212 provides for coupling of dual TM₅₂₉ modes in the resonator 204. Ahigh dielectric constant structure can support more than one bound modesimultaneously, either as degenerate (i.e., field orthogonal, butresonant, at the same frequency) or as separate modes separated in thefrequency domain. Consequently, multimode filter configurations areattainable, as depicted in FIG. 10.

As depicted in each of FIGS. 4-10, the structure of the apparatus'senclosure is too small to be resonant at frequencies at or below that ofthe high dielectric constant structure. Consequently, the enclosure isnot a fundamental resonator in itself, but rather is below cutoff (i.e.only propagation of evanescent modes is possible within the enclosure,external to the high dielectric constant structure).

A composite resonator element 300, according to an alternativeembodiment of the invention, is illustrated in FIGS. 11a-11 c. The sizeand shape of resonator 300 is essentially the same as the preferredembodiment 10 shown in FIGS. 3a-3 c except with the metallic anddielectric materials reversing roles and positions. Accordingly, thealternative resonator 300 includes a metallic core 312 surrounded by adielectric layer 314 to form a “doughnut” or “hockey puck” shape. Themetallic core includes a top surface or face 322, a bottom surface orface 324 and an interior sidewall surface or face 326. As best seen inFIG. 11b, the circumference 326 of metallic core 312 is surrounded by asidewall dielectric band or layer 318. The dielectric layer 314 is,therefore, composed of the sidewall band or layer 318.

EXAMPLE 1

For comparison purposes, a calculation was made with the standardTrans-Tech Dielectric Resonator Design package (available fromTrans-Tech, 552 Adamstown Road, Adamstown, Md. 21710) for a conventionalprior art resonator with an ∈_(r)=80 to obtain a desired frequency of0.733 GHz. The ultimate dielectric required a width of 1.940″ by 0.873″.The volume then is πr²h=2.58 in³.

In contrast, using commercial available Mathcad™ 7 program distributedby MathSoft, Inc., 101 Main Street, Cambridge, Mass. 02142, thefollowing calculations were obtained:

Structure Inputs: radius in inches a: = .784 height in inches d: = .63relative permittivity of dielectric ε_(r): = 80 conductivity of metalmet: = 3 metal = 1 aluminum .3817 = 2 silver .6173 = 3 copper .58relative permittivity of metal μmet: = .9999736 cut plane distance zd: =1 d (decimal percentage of total height) Field Plot Inputs: Choose avalue [0, 1] for TE: TE: = 1 TE = 1 for TE calculations TE = 1 for TMcalculations $\begin{matrix}{{check}:={\begin{matrix}{{{``{{Enter}\quad {in}\quad a\quad 0\quad {or}\quad 1\quad {value}\quad {only}}"}\quad {if}\quad {TE}} > 1} \\{{``{okay}"}\quad {otherwise}}\end{matrix}}} \\{{check} = {``{okay}"}}\end{matrix}$

Choose Mode number N is the number of circumferential variations in thefield N: = 0 M: = 1 L: = 1 M is the number of radial variations L is thenumber of axial variations ${check}:={\begin{matrix}{{{``{{{Enter}\quad {in}\quad a\quad L}>=1}"}\quad {if}\quad L} < 1} \\{{{``{{{Enter}\quad {in}\quad a\quad M} > 1}"}\quad {if}\quad M} < 1}\end{matrix}}$

TE_(nml) = root_(NM) $\begin{matrix}{{{``{{{Enter}\quad {in}\quad a\quad M}>=1}"}\quad {if}\quad M} < 1} \\{{``{okay}"}{\quad \quad}{otherwise}}\end{matrix}$

TE_(nml) root_(NM) check = “okay” TM_(φrz) = root_(NM) Define cutplanefor field plots: Option 1 - φ cut with φ = 90° option: = 3 Option 2 - φcut with φ = 0° Option 3 - Z cut with 0 < z < d Constants: ε0: =8.854187817 · 10⁻¹² μ0: = 4 · π · 10⁻⁷ j: = {square root over (−1)}$c:={{\frac{1}{\sqrt{\mu \quad {0 \cdot ɛ}\quad {0 \cdot ɛ}\quad r}}\quad c} = {{{3.352 \cdot 10^{7}}\quad \eta}:=\sqrt{\frac{\mu \quad 0}{ɛ\quad {0 \cdot ɛ}\quad r}}}}$

Calculate Bessel function:${{guess}\quad \left( {n,r} \right)}:={{\pi \quad \left( {r + \frac{n}{2} - \frac{1}{4}} \right)} - \frac{{4 \cdot n^{2}} - 1}{8 \cdot \left\lbrack {\pi \cdot \left( {r + \frac{n}{2} - \frac{1}{4}} \right)} \right\rbrack}}$

TOL: = 10⁻⁸${{jn}\left( {n,x} \right)}:={{{{root}\left( {{{Jn}\left( {n,x} \right)},x} \right)}\quad\begin{bmatrix}P_{01} & P_{02} & P_{03} \\P_{11} & P_{12} & P_{13} \\P_{21} & P_{22} & P_{23}\end{bmatrix}} = \begin{bmatrix}2.405 & 5.520 & {\quad 8.652} \\3.832 & 7.016 & 10.176 \\5.135 & 8.417 & {11.62\quad}\end{bmatrix}}$

jroot(n,r): = jn(n,guess(n,r)) range variables:  n: = 0 . . . 4  m: = 1. . . 4 roots_(nm): = jroot(n,m) roots_(NM) = 2.405${roots} = \begin{bmatrix}0 & 2.405 & {5.52\quad} & {\quad 8.654} & 11.792 \\0 & 3.832 & 7.016 & {10.173\quad} & 13.324 \\0 & 5.136 & 8.417 & {11.62\quad} & 14.796 \\0 & {6.38\quad} & 9.761 & {13.015\quad} & 16.223 \\0 & 7.588 & {11.065\quad} & {14.373\quad} & 17.616\end{bmatrix}$

${{guess}\quad \left( {n,m} \right)}:={{\pi \cdot \left( {m + \frac{n}{2} - \frac{1}{4}} \right)} - \frac{{4 \cdot n^{2}} - 1}{8 \cdot \left\lbrack {\pi \cdot \left( {m + \frac{n}{2} - \frac{1}{4}} \right)} \right\rbrack}}$

TOL: = 10⁻³${j^{\prime}{n\left( {x,n} \right)}}:={{{root}\quad \left( {{\frac{}{x}{{Jn}\left( {n,x} \right)}},x} \right)\quad j^{\prime}0(x)}:={{root}\left( {{{- J}\quad 1(x)},x} \right)}}$

j′root(n,m) := if(n = 0,j′0(guess(1,m)),j′n(n,guess(n − 1,m)))roots′_(n,m): = j′root(n,m) ${roots}^{\prime} = {{\begin{pmatrix}0 & 3.832 & 7.016 & 10.173 & 13.324 \\0 & 1.841 & 5.332 & {\quad 8.535} & 11.705 \\0 & 3.056 & 6.707 & 9.97 & 13.168 \\0 & 4.199 & 8.016 & 11.346 & 14.581 \\0 & 5.318 & 9.285 & 12.682 & 15.964\end{pmatrix}\quad {roots}_{N,M}^{\prime}} = 3.832}$

$p_{n,m}:={{\frac{{roots}_{n,m}}{acm}\quad p_{n,m}^{\prime}}:=\frac{{roots}_{n,m}^{\prime}}{acm}}$

Part 1: Calculate Cutoff Frequency: ${fcut}:={\begin{matrix}{{\frac{{roots}_{N,M}^{\prime}}{{2 \cdot \pi \cdot {acm}}\quad \sqrt{\mu \quad {0 \cdot ɛ}\quad {0 \cdot ɛ}\quad r}}\quad {if}\quad {TE}} = 1} \\{{\frac{{roots}_{N,M}^{\quad}}{2 \cdot \pi \cdot \quad \sqrt{\mu \quad {0 \cdot ɛ}\quad {0 \cdot ɛ}\quad r}}\quad {if}\quad {TE}} \neq 1}\end{matrix}}$

Part 2: Calculate Resonant Frequency: $\begin{matrix}{{fres}:={\quad \begin{matrix}{{\left\lbrack {\frac{c}{4 \cdot \pi}\sqrt{\left( \frac{{roots}_{N,M}^{\prime}}{acm} \right)^{2} + \left( \frac{L \cdot \pi}{dcm} \right)^{2}}} \right\rbrack \quad {if}\quad {TE}} = 1} \\\left\lbrack {{\left. \frac{c}{4 \cdot \pi}\sqrt{\left( \frac{{roots}_{N,M}}{acm} \right)^{2} + \left( \frac{L \cdot \pi}{dcm} \right)^{2}} \right\rbrack\quad {if}\quad {TE}} \neq 1} \right.\end{matrix}}} \\{{{fres} = {7.332 \cdot 10^{8}}}\quad}\end{matrix}$

Here the volume is πr²h=1.21 in³. Therefore, the metal ring resonator 10has 1.21/2.58=47% of the volume of a conventional all dielectricresonator as shown in FIG. 1 with the same ∈r=80.

While the invention has been described with reference to the preferredembodiment thereof, it will be appreciated by those of ordinary skill inthe art that various modifications can be made to the structure andfunction of the individual parts of the system without departing fromthe spirit and scope of the invention as a whole.

What is claimed is:
 1. A resonator apparatus that resonates in asubstantially bound mode for use inside of a structure having at leastone cavity, said resonator comprising: a dielectric core having anexterior surface with at least two faces; a metallic layer coveringsubstantially all of at least one face; means for producing adisturbance on one said exterior faces of said dielectric core; whereinsaid resonator resonates in a substantially bound mode and wherein saidresonator resonates at least two peak frequencies and wherein saidresonant frequency is below the normal cutoff resonant frequency of saidcavity.
 2. A resonator apparatus that resonates in a substantially boundmode for use inside of a structure having at least one cavity, saidresonator comprising: a dielectric core having an exterior surface withat least two faces; and, a metallic layer covering substantially all ofat least one face; wherein said resonator resonates in a substantiallybound mode and wherein none of said at least one cavity is a fundamentalresonator.
 3. A resonator apparatus comprising at least a firstresonator that resonates in a substantially bound mode for use inside ofa structure having at least one cavity, said first resonator comprising:a dielectric core having an exterior surface with at least two faces;and, a metallic layer covering substantially all of at least one face;wherein said first resonator resonates in a substantially bound mode andwherein said dielectric core has a substantially rectangular shapehaving at least four corners and wherein at least one of said cornersincludes a notch for coupling dual TM₁₁ modes and wherein said resonatorresonates in at least two modes.